Fine materials generated from mining activities are often found well-dispersed in aqueous environments, such as wastewater. The finely dispersed materials may include such solids as various types of clay materials, recoverable materials, fine sand and silt. Separating these materials from the aqueous environment can be difficult, as they tend to retain significant amounts of water, even when separated out, unless special energy-intensive dewatering processes or long-term settling practices are employed.
An example of a high volume water consumption process is the processing of naturally occurring ores, such as coal and oil sands. Naturally occurring ores are heterogeneous mixtures of hydrophobic organic molecules and solid inorganic matter. During the processing of such ores, colloidal particles, such as clay and mineral fines, are released into the aqueous phase often due to the introduction of mechanical shear associated with the hydrocarbon-extraction process. In addition to mechanical shear, alkali water is sometimes added during extraction, creating an environment more suitable for colloidal suspensions. A common method for disposal of the resulting “tailing” solutions, which contain fine colloidal suspensions of clay and minerals, water, sodium hydroxide and small amounts of remaining hydrocarbon, is to store them in “tailings ponds”. These ponds take years to settle out the contaminating fines, making the water unsuitable for recycling.
Certain industrial processes that use a large volume of water place strains on the local water supply. For example, in the oil sands extraction industry, flow rate decreases have been observed in the nearby rivers from which processing water is drawn. As a specific example, water demands are a great concern in Athabasca, an oil sand deposit located in northern Alberta Canada, near the Athabasca River. Oil sands from the Athabasca deposit are being mined and processed at a rate of roughly 1,000 kilotonnes per day. The water demand to process this amount of ore adds up to roughly 500 kdam3 per year, accounting for 8% of the province's water usage. With the vast majority of the water ending up in tailings ponds, eventually the surrounding rivers will no longer be able to sustain the industry's water demand. An effective and efficient method of recycling water in this industry is essential for its long-term viability.
In addition, certain industrial processes can create waste streams of large-particle inorganic solids. Using the oil sands example, inorganic solids such as quartz remain after the extraction of hydrocarbon from the oil sands ore. Since the ore that is processed only contains about 8-12% desirable hydrocarbon, a large amount of large-particle inorganic material remains after hydrocarbon extraction. This residue is typically removed in initial separation phases of processing due to its size, insolubility and ease of sequestering. Disposal or storage of this waste material has become a problem for the oil sands industry, again due to the vast volume of many industrial processes. Attempts have been made to use this large coarse solid as a flocculant ballast for the fine colloidal suspension in the tailings ponds; however, aggregation has not been sustainably observed. It would be advantageous to modify this material so that it could be useful in-situ for wastewater treatment purposes.
A typical approach to consolidating fine materials dispersed in water involves the use of coagulants or flocculants. This technology works by linking together the dispersed particles by use of multivalent metal salts (such as calcium salts, aluminum compounds or the like) or high molecular weight polymers such as partially hydrolyzed polyacrylamides. With the use of these agents, there is an overall size increase in the suspended particle mass; moreover, their surface charges are neutralized, so that the particles are destabilized. The overall result is an accelerated sedimentation of the treated particles. Following the treatment, though, a significant amount of water remains trapped with the sedimented particles. These technologies typically do not release enough water from the sedimented material that the material becomes mechanically stable. In addition, the substances used for flocculation/coagulation may not be cost-effective, especially when large volumes of wastewater require treatment, in that they require large volumes of flocculant and/or coagulant. While ballasted flocculation systems have also been described, these systems are inefficient in sufficiently removing many types of fine particles, such as those fine particles that are produced in an oil sands mining.
Particular needs exist in the oil sands industry for removing suspended particles from fluid solutions. Tailings flowing directly from processing oil sands (termed “whole tailings”) can contain fine clay particles (termed “clay fines”) suspended in an alkaline water solution, along with suspended sand and other particulate matter. The whole tailings can be separated into two fluid streams by processes such as cycloning, where one fluid stream (called the underflow) contains sand, and the other fluid stream (called the overflow) contains the suspended fine clay particles. The overflow from cycloning that contains the fine clay particles is termed fine tailings. Fine tailings can be directed to large man-made tailings ponds to allow the clay particles to settle out gradually via gravity. The settling process can take many years. Tailings ponds typically have four layers, including a bed of settled sand, an overlying thick liquid layer called “mature fine tailings,” a liquid layer bearing suspended fines, and a supernatant layer of clarified water. This watery top layer, chilled by exposure to the ambient air temperature, can be reused for oil sands processing, but it must be heated up to the processing temperature of approximately between 50°-80° C. before it can be used.
There remains an overall need in the art, therefore, for a treatment system that removes suspended particles from a fluid solution quickly, cheaply, and with high efficacy. It is also desirable that the treatment system yield a recovered (or recoverable) solid material that retains minimal water, so that it can be readily processed into a substance that is mechanically stable, potentially capable of bearing weight or supporting vehicular traffic, i.e., “trafficable.” It is further desirable that the treatment system yield a clarified water that can be readily recycled for further industrial purposes.
As applied to the oil industry, it is desirable that the whole tailings be processed before being directed to the tailings ponds so that the water is separated from the suspended solids. If this separation is performed soon after oil sands processing, the recovered water will still be hot, so that there can be conservation of energy needed to heat the recycled water to the processing temperature. Furthermore, processing the whole tailings to recover water and solids can decrease the amount of waste materials that must be stored in facilities like tailings ponds.
An additional need in the art pertains to the management of existing tailings ponds. In their present form, they are environmental liabilities that may require extensive clean-up efforts in the future. It is desirable to prevent their expansion. It is further desirable to improve their existing state, so that their contents settle more efficiently and completely. A more thorough and rapid separation of solid material from liquid solution in the tailings pond could allow retrieval of recyclable water and compactable waste material, with an overall reduction of the footprint that they occupy.